1. Field of the Invention
The invention relates to electrochemistry and, more particularly, to electrochemical sensors.
2. Background Information
Electrochemical sensors are used in an expansive variety of applications to measure parameters of importance to chemical and physical processes of all kinds, from biochemical to industrial. A variety of electrochemical measurements are of frequent interest. For example, many chemical processes, including biochemical processes, optimally occur at a particular pH; monitoring the pH of a solution involved in the process thus frequently provides important information as to the process and can signal departures from normal. Another type of measurement that is frequently made is the conductivity of a solution. Conductivity measurements provide information as to the ionic strength of the solution, and are often important in their own right as well as in providing information as to the effect of conductivity on chemical reactions. In addition to these basic characteristics, measurements of characteristics such as the oxidation/reduction potential of a reaction occurring at specific electrodes provide information as to the nature of the constituents taking part in reactions.
Typically, electrochemical sensors are specialized for the particular parameter being measured. Thus, pH is usually determined by means of a potentiometric sensor that commonly includes a glass membrane that is selective for the H+ ion. Acidity or alkalinity is usually determined by a titration, i.e., by adding aliquots of a base in the case of an acid solution, and of an acid in the case of a basic solution, until the preexisting acid or base in the solution is entirely neutralized. A sensor, which may be a pH sensor, is used to determine the point at which balance is achieved. Conductance of a solution is typically determined by yet another instrument, which applies a voltage across a measured length of the solution and which measures the resultant current. Thus, to make several types of measurements on a particular solution typically requires the use of several different instruments.
Semiconductor chips have heretofore been used as sensor elements in electrochemical sensors. One such element is a Field Effect Transistor, specifically, an Ion-Selective Field Effect Transistor (ISFET). In such a device, the transistor is formed with the usual gate, source, and drain electrodes, with the potential applied to the gate controlling the conductivity between the source and the drain. In contrast to conventional FET devices, however, the gate electrode is left exposed and thus can be placed directly in contact with a liquid solution when the chip is immersed in such a solution. The gate is thus made responsive to the electrochemical potential of the solution in its neighborhood, and the current flow between the transistor source and drain provides an indication of this potential.
A device of the type described above has been known for use as an electrochemical sensor in an aqueous solution. See van der Schoot and Bergveld, Sensor and Actuators, vol. 8 (1985), pp. 11-22. In this application, a “generating electrode” in the form of a thin layer of a noble metal such as gold or platinum is deposited around the exposed gate of the ISFET. A counter electrode and a reference electrode are likewise deposited on the chip substrate, in the vicinity of the generating electrode. The electrodes are of substantial area. Current pulses applied between the generating electrode and the counter electrode generate titrant for the solution in which the sensor is immersed. If the current is made anodic at the generating electrode, H+ ions are generated at the location of the gate, and these ions titrate a basic solution. Conversely, if the current is made cathodic at the generating electrode, OH− ions are generated at the location of the gate, and these ions titrate an acidic solution. In either case, the end point is detected by the change in pH at the ISFET gate.